In total hip replacement the head and most of the neck of the proximal femur are removed and a femoral prosthesis is implanted. The conventional approach in designing a femoral prosthesis has assumed that loads should be transferred between the prosthesis and the femur over the entire length of the part of the prosthesis that is received within the femur, which includes, of course, the stem that extends a relatively large distance into the femoral shaft. The distribution of the load transfer depends on the specific design of the prosthesis and the type of fixation.
In prostheses that are implanted with bone cement (polymethyl methacrylate), the loads are transferred from the prosthesis through the cement to the endosteal surface of the bone throughout the entire cement layer, but the distribution of stress is non-uniform, though it can be predicted using stress analysis techniques such as the finite element method. In prostheses that are implanted without bone cement, sintered porous metal coatings on the prosthesis promote ingrowth of bone for biological retention, and the distribution of the load transfer from the prosthesis to the endosteal surface of the bone depends upon the apposition of the bone, the extent of bone ingrowth and the design of the prosthesis. In presently known designs of non-cemented femoral prostheses the apposition and, hence, the regions where load transfer will occur are unpredictable.
Bone remodels according to the loads applied to it, and the long term stress distribution in the bone is, therefore, of great importance. A beneficial effect of the remodeling property of bone is, of course, that the structural characteristics of regions of the bone that are loaded by the prosthesis and by the tendons at the hip joint are likely to be enhanced. Conversely, regions of the bone where the loads are decreased deteriorate through loss of bone structure. It is, therefore, important that a prosthesis provide for load transfer that is predictable and that minimizes the possibility of unloading of the bone.
Inherent in known designs of femoral hip joint prostheses of both the cemented and non-cemented types is some degree of unloading of the bone in the region of the stem of the prosthesis, inasmuch as the stem is coupled to the bone to a lesser or greater extent, depending on the design. When the bone bends, the stem has to bend, which produces stress in the stem, stress (load) that would otherwise be borne by the bone. Bending of the stem also alters the load transfer between the bone and the prosthesis at the proximal region, tending to unload the proximal medial aspect of the femur, a region where the bone is usually under a large axial compressive load, and load the proximal lateral aspect, a region where the bone is under tension. Bone resorption at the proximal femur reduces load transfer in that region and increases the load transfer in the stem. Loosening of the implant and stem fracture can result.